1. Field of the Invention
The present invention relates to a semiconductor device which can store data.
In this specification, a semiconductor device generally means a device which can function by utilizing semiconductor characteristics, and an electro-optic device, a semiconductor circuit, and an electronic device are all semiconductor devices.
2. Description of the Related Art
A memory element generally has a structure in which two electrodes as two terminals of the memory element are provided above and below a dielectric layer.
In Patent Document 1 (Japanese Patent Laid-Open No.: 2002-26277), a memory device is proposed, in which electrodes as two terminals of an element are provided above and below a layer containing an organic compound, voltage is applied to the electrodes to cause short-circuit, and information is stored by setting an initial state to be “0” and a conducted state to be “1”, and a method for driving the memory device is also proposed.
As a memory circuit provided in the semiconductor device, a DRAM (Dynamic Random Access Memory), an SRAM (Static Random Access Memory), an FeRAM (Ferroelectric Random Access Memory), a mask ROM (Masked Read Only Memory), an EPROM (Electrically Programmable Read Only Memory), an EEPROM (Electrically Erasable and Programmable Read Only Memory), a flash memory, and the like are given. Among them, a DRAM and an SRAM are volatile memory circuits in which data are erased when power is turned off; therefore, it is necessary to write data every time the power is turned on. An FeRAM is a nonvolatile memory circuit, and the number of manufacturing steps thereof is increased because a capacitor including a ferroelectric layer is used in an FeRAM. A mask ROM has a simple structure; however, it is necessary to write data during manufacturing steps thereof, and data cannot be additionally written. An EPROM, an EEPROM, and a flash memory are nonvolatile memory circuits; however, there is a problem that the number of manufacturing steps thereof is increased because an element having two gate electrodes is used in an EPROM, an EEPROM, and a flash memory.
In addition, a memory circuit such as a DRAM using an inorganic material for a dielectric substance, stores two values by presence or absence of charge stored in a capacitor.
On the other hand, in a memory circuit using an organic compound for a dielectric substance, a memory element is formed by providing an organic compound between a pair of upper and lower electrodes. However, when the electrode is formed over a layer containing an organic compound, a temperature is limited because the layer containing an organic compound can be influenced depending on a temperature for forming the electrode. A forming method for the electrode is limited due to this limitation of a temperature and an expected electrode cannot be formed. Therefore, there are problems that miniaturization of an element is inhibited. A problem caused by an electrode formed over a layer containing an organic compound is required to be solved from an aspect of inhibition of element miniaturization.
In addition, in a case of an organic memory element where a pair of electrodes as two terminals is formed above and below a layer containing an organic compound, a pair of electrodes is required to be formed by a plurality of steps because a pair of electrodes is provided above and below the layer containing an organic compound. Therefore, there is a problem that a manufacturing process becomes complicated. The complicated manufacturing process is a problem that is required to be solved from an aspect of a manufacturing cost.